The present invention relates to an optical pulse transmission system and an optical pulse transmitting method which permit high-speed transmission of electric signal pulses after converting them into optical pulses. The present invention also pertains to a waveform converting method advantageous for use at the transmitting side for conversion of the electric signal pulses to optical pulses and a light intensity modulator advantageous for use at the transmitting side for the modulation of light, and an optical pulse detecting method advantageous for use at the receiving side for the detection of the optical pulses transmitted thereto.
Many of semiconductor device testing apparatus (commonly called IC testers) for testing various semiconductor devices, including semiconductor integrated circuits (ICs), for instance, employ a semiconductor device transporting and processing or handling apparatus (commonly called a handler) which transports semiconductor devices for testing and sorts out tested semiconductor devices based on the test results. In the semiconductor device testing apparatus of the type that has the semiconductor device transporting and processing or handling apparatus (hereinafter referred to as handler) connected thereto, a test head for applying a test signal of a predetermined pattern to a semiconductor device under test (commonly called DUT) is placed in a test section of the handler away from the main body (tester proper) of the semiconductor device testing apparatus. The test head and the tester proper are connected via an electric signal transmission line such as a cable, through which the test signal of a predetermined pattern is fed from the tester proper to the test head, and the test signal is applied to the semiconductor device under test via a socket mounted on the test head. A response signal from the semiconductor device under test is sent over the electric signal transmission line from the test head to the tester proper for measuring electrical properties of the semiconductor device.
In recent years, semiconductor integrated circuits (hereinafter referred to as ICs) are becoming faster and the number of terminals (pins) mounted on the package is also on the increase. As a result, the transmission of an electric signal over a transmission line such as a cable as in the above-mentioned semiconductor device testing apparatus will cause such defects as listed below.
(1) With the use of the cable or similar electric wire, there is a limit to the frequency of the electric signal for transmission therethrough, and an increase in the signal frequency is likely to cause the degradation of the signal waveform. This imposes severe limitations on the signal transmission rate, making it difficult to test fast-acting ICs.
(2) Cables now in use are so thick that an increase in the number of cables with an increase in the number of IC terminals will inevitably make thick, heavy and hence hard-to-handle the bundle of cables between the tester proper and the test head.
As a solution to the above-mentioned problems, an optical transmission system has recently come into use which is excellent in the signal transmission rate and in the frequency characteristic as compared with the above-mentioned electric transmission system and is capable of employing, as its transmission medium, an optical fiber or like transmission line which is thin and light. Next, a common optical transmission system will be described.
For the generation of a binary digital signal (optical pulses) by modulating light, a light intensity modulation system which changes the intensity of light according to an information signal (a modulation signal) is used in most instances because of simplicity of the modulation techniques involved. Usually, the light intensity modulation system has a configuration in which a laser diode capable of fast light intensity modulation is provided as a light emitting device at the transmitting side, a fast-response photodiode is provided at the receiving side and an optical fiber is used as a transmission medium; optical pulses emitted from the laser diode at the transmitting side are sent over the optical fiber to the receiving side, where the optical pulses sent thereto are converted by the photodiode to electric signals.
FIG. 23 is circuit diagram schematically depicting an example of a conventional optical transmission system using an optical transmission line. The illustrated optical transmission system comprises an optical pulse transmitting device 101, an optical pulse receiving device 102, and an optical transmission line 109, such as an optical fiber, for interconnecting the transmitting device 101 and the receiving device 102.
The optical pulse transmitting device 101 is provided with a main circuit 103 for outputting an electric pulse signal to be sent to the receiver side, a driving circuit 104 connected at its input terminal to an output terminal 103A of the main circuit 103, and a semiconductor laser or similar light emitting device 105 connected between the output terminal of the driving circuit 104 and a common conductor. The light emitting device 105 responds to an electric pulse signal fed thereto from the driving circuit 104 to emit light and hence generate optical pulses, which are provided via an optical connector 109A onto the optical transmission line 109 for transmission to the optical pulse receiving device 102.
The optical pulse receiving device 102 comprises a photodiode or similar photodetector 106, a detecting circuit 107 connected at its input terminal to the output terminal of the photodetector 106, and a main circuit 108 connected at its input terminal to the output terminal of the detecting circuit 107; the optical pulses sent over the optical transmission line 109 are input into the photodetector 106 via an optical connector 109B. The photodetector 106 converts the received optical pulses to an electric pulse signal, and applies it to the detecting circuit 107. The detecting circuit (usually formed by a current-to-voltage converting amplifier) 107 detects the electric pulse signal fed thereto and provides it to the main circuit 108. The main circuit 108 performs various processes based on the electric pulse signal input thereto.
In general, a laser diode is used as the light emitting device 105, but as is well-known in the art, the laser diode has a defect that the quantity of light emitted therefrom varies with a temperature change. FIG. 24 shows injected current vs. output light power characteristics of the laser diode. In FIG. 24 the curve A indicates the injected current vs. output light power at a temperature T1 (xc2x0 C.) and the curve B the injected current vs. output light power at a temperature T2 (xc2x0 C.) (where T1 less than T2).
As is evident from FIG. 24, current values ION1 and ION2 for driving the laser diode to emit light vary with ambient temperature. As a result, if a driving current ID of the same peak value as the above-mentioned current values is used to drive the light emitting device 105 by the driving circuit 104, the light emitting device 105 outputs an optical pulse OP1 at the temperature T1 (xc2x0 C.) and an optical pulse OP2 at T2 (xc2x0 C.) as depicted in FIG. 24.
As will easily be understood from FIG. 24, an ambient temperature variation will cause a change in the power of the optical pulse that is output from the light emitting device 105. Hence, in the case of receiving the optical pulses OP1 and OP2 by the optical pulse receiving device 102, optical pulse waveforms, which cross a threshold voltage EC for detecting the reception of the optical pulses, lag in timing according as peak values of the received signals are large or small, as indicated by xcex94t1 and xcex94t2 in FIG. 25. This entails a disadvantage that the temperature variation is sent as jitter to the receiving device 102.
A real-world example in which the generation of jitter is disadvantageous can be found, for example, in the application of the above-mentioned optical transmission system to the semiconductor device tester. As referred to above, in the semiconductor device tester the test head with a socket mounted thereon is placed apart from the tester proper. The test head comprises a driver for applying a test signal of a predetermined pattern or the like to the semiconductor device under test, a comparator for receiving a response output signal from the semiconductor device under test and for making a logical-level decision; furthermore, the test head serves as an interface with the semiconductor device. And there are provided many signal transmission lines between the tester proper and the test head.
Where these signal transmission lines are each formed by an optical transmission line like an optical fiber to transmit a high-speed signal (an optical pulse), the optical transmission line 109 needs to have a multi-channel structure. In a system configured to transmit and receive optical signals over such a multi-channel optical transmission line, a temperature change causes jitter in the pulse being transmitted; furthermore, when the amount of jitter differs for each channel, a timing error is induced between optical signals sent over the transmission lines of the respective channels, making it impossible to test semiconductor devices (ICs) under normal conditions.
In FIG. 26 there is depicted an example of a light intensity modulator for use in such an optical transmission system as mentioned above. This light intensity modulator is equipped with an input side comparator 200 which receives the signal voltage of a digital input signal (an electric pulse signal) and a threshold voltage and compares these voltages, a current switching circuit 201 which turns ON/OFF in accordance with the result of comparison by the input side comparator 200, and a semiconductor laser 202 which is driven on the basis of a current waveform resulting from the ON/OFF operation of the current switching circuit 201. The current switching circuit 201 comprises a pair of transistors TR1 and TR2 having their emitter connected in common, and a pair of transistors 203 and 205 having their bases connected in common. The collectors of the pair of transistors TR1 and TR2 are connected to terminals of the semiconductor laser 202 corresponding thereto, respectively, and their common-connected emitter is connected to the collector of the transistor 203.
In the light intensity modulator of the above construction, when the right-hand transistor TR2 of the pair of emitter-common transistors TR1 and TR2 forming the current switching circuit 201 is ON, current controlled by the transistor 203 in advance is injected into the semiconductor laser 202, which generates an optical output of a level corresponding to the magnitude of the injected current. Incidentally, a DC bias current necessary for driving the semiconductor laser 202 is controlled by a transistor 204 which has its collector connected to a current injection terminal of the semiconductor laser 202.
By generating a binary optical signal, i.e. optical pulses by the above light intensity modulator, it is possible to implement an optical transmission system which transmits optical pulses at high speed. For example, in the aforementioned semiconductor device tester, however, pulses of many different periods are mixed on the optical signal transmission line between the tester proper and the test head; moreover, very high precision of timing is required in the modulation of light. Accordingly, the application of the above-mentioned optical transmission system to the semiconductor device tester may encounter such problems as mentioned below.
(1) In general, the light intensity is unstable in level (the low-frequency component fluctuates greatly); therefore, when a binary optical signal is discriminated using a fixed threshold level at the receiving side as depicted at the lower part in FIG. 27, errors occur in data (0, 1) and timing as shown. The upper waveform in FIG. 27 shows an electric pulse to be sent from the transmitting side.
(2) The rise time (a light emission delay time) of such a light emitting device as a semiconductor laser varies with the device temperature, and in general, the rise time differs from one device to another. Hence, the light emission delay time differs with temperature changes or with devices as shown in FIG. 28. The difference in the light emission delay time constitutes a factor in causing the above-mentioned timing error.
To solve the above problem (1), there have been proposed a method that effects temperature control to keep the device temperature constant and a method that monitors the light intensity and maintains the device output at a fixed level (to stabilize the light intensity). However, either method involves an expensive transmission module, and hence presents a problem in terms of cost although it can be implemented in a semiconductor device tester or the like which requires a number of transmission lines. Additionally, the stabilization of light intensity is hard to perform in the case of transmitting optical pulses at high speed.
As a solution to the above problem (2), there has been proposed a method that does not provide the binary optical signal by the emission and extinction of light by the light emitting device but instead, as depicted in FIG. 29, drives the light emitting device in a state of emitting light of a certain level (offset light) at all times and generates the binary optical signal according as the light intensity varies from the offset light. With this method, since the light emitting device emits light at all times, the light emission delay time is less affected by the temperature change and is not likely to differ from one device to another. However, the SN ratio lowers because the difference in light intensity between the binary data xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d. In addition, since the both pieces of data xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d of the binary signal is affected by the light intensity variation, the settlement of the above-mentioned problem (1) becomes increasingly important.
In a multi-channel transmission module for use in a technical field which requires a number of transmission line, such as an ATM (Asynchronous Transmission Mode) exchange, there is adopted a method that the receiving side extracts only a proper AC component of the optical signal (AC coupling) and discriminates the binary signal based on the threshold level set at 0 V as depicted in the lower portion of FIG. 30. Incidentally, the upper waveform in FIG. 30 shows the electric pulse signal to be transmitted from the transmitting side.
With this method, it is true that timing and data errors can be reduced relatively easily. But if the ratio between the binary data xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d inclines toward one particular data value, the threshold level also shifts towards that data value, resulting in the occurrence of a timing error. It is a matter of course that DC-wise data fixed for a long time cannot be discriminated, besides it is also impossible to detect which data value continues for a long time.
In other words, according to the above AC coupling method using the threshold level set at 0 V, when the data value is left in a fixed state (for example, in a state in which no signal is present), a low-level fluctuation by noise caused in that state will be detected erroneously as one of the data values of the binary signal. Thus this method has a shortcoming that the data values of the binary signal need to vary at all times so as to prevent the above-mentioned erroneous detection. Accordingly, this method cannot be employed in a system in which signals of many different periods are mixed and the data value of the binary signal inclines heavily toward one particular value (0 or 1), that is, in a system in which the DC component is present and the timing accuracy is regarded as important, for example, as in the case of transmitting signals between the tester proper and the test head in the semiconductor device tester.
Further, there has also been proposed such a method as depicted in FIG. 31. With this method, a rise edge and a fall edge of a binary electric signal are detected, then a pulse signal corresponding to the detection of each edge is generated, followed by a pulse signal having its polarity inverted, that is, upon detection of the rise edge, a positive pulse signal is generated and a negative pulse signal having its polarity inverted is continuously generated, thereby to form a pair of pulses having their polarities opposed to each other (opposite-polarity pulse pair), and upon detection of the fall edge, a negative pulse signal is generated and a positive pulse signal having its polarity inverted is continuously generated, thereby to form a pair of pulses having their polarities opposed to each other (opposite-polarity pulse pair). Based on these opposite-polarity pulse pairs, a semiconductor laser is driven to create similar opposite-polarity optical pulse pairs which are transmitted to the receiving side.
With this method, since the transmitted optical pulse pairs are optical signals indicating individual timing of the rise and fall of the binary electric signal to be transmitted, the receiving side can regenerate the original binary electric signal by receiving the timing-indicating optical signals and discriminating between the rise and the fall timing of the original signal. Hence, this method can satisfactorily be applied even to the system in which signals of many different periods are present and the data value of the binary signal inclines heavily toward a particular value (0 or 1) as in the case of transmitting signals between the tester proper and the test head in the semiconductor device tester.
That is to say, since the receiving side needs only to receive opposite-polarity optical pulse pairs as timing signals concerning the rise and fall of the original signal, there is no possibility of the threshold level inclining toward a particular one of the data values, causing an error in timing or in data value. Besides, it is also possible to accurately discriminate the DC-wise data value fixed for a long time.
In FIG. 32 there is depicted an example of a conventional driving circuit which detects the leading and fall edges of a binary electric signal, then generates an opposite-polarity pulse pair upon detection of each edge, and drives a semiconductor laser accordingly.
The driving circuit is provided with: an OR circuit 300 which receives at one input terminal a binary electric signal to be sent; a first inverting circuit (an inverter) 301 which inverts the polarity of the binary electric signal to be sent; a first delay circuit 302 which delays the output signal from the inverter 302 by a predetermined time interval and applies the delayed signal to the other input terminal of the OR circuit 300 and one input terminal of an AND circuit 303; a second inverting circuit (an inverter) 304 which inverts the polarity of the output signal from the delay circuit 302; and second delay circuit 305 which delays the output signal from the inverter 304 by a predetermined time interval and applies the delayed signal to the other input terminal of the AND circuit 303. Incidentally, the output signals from the OR circuit 300 and the AND circuit 303 are applied, in opposite polarity, to a semiconductor laser 312.
With the driving circuit of the above construction, it will easily be understood that, as shown in FIG. 33, a positive-logic pulse waveform (d) and a negative-logic pulse waveform (e) are generated from leading and fall edges (a) to (c) of an input binary electric signal and are added together to form opposite-polarity pulse pairs (f). Based on the opposite-polarity pulse pairs (f), the semiconductor laser 312 is driven to generate pairs of opposite-polarity optical pulses as depicted in the lower part of FIG. 31.
In the generation of the opposite-polarity pulse pairs (f) by adding the positive-logic pulse waveform (d) and the negative-logic waveform (e) derived from the leading and fall edges of the binary input electric signal, the polarity inverting portion of each opposite-polarity pulse pair forms a joint between the positive-logic waveform (d) and the negative-logic waveform (e). Hence, in the conventional driving circuit, the polarity inverting portion of the opposite-polarity pulse pair required to be highly accurate may sometimes form a discontinuous edge, incurring the possibility of timing accuracy being impaired.
A first object of the present invention is to provide an optical transmission system and method which can obviate the above-described problems of the prior art.
According to a first aspect of the present invention, there is provided an optical pulse transmitting method in which an electric pulse is applied to a light emitting device provided at the transmitting side, thereby emitting an optical pulse from the light emitting device, which is, in turn, transmitted to the receiving side over an optical transmission line, and the transmitted optical pulse is converted into an electric pulse by a light receiving device provided at the receiving side, which is acquired as a received signal, and the method is characterized in that: in the transmitting side, a symmetrical waveform signal which changes symmetrically in positive side and in negative side with a DC bias current value as its center is applied as the electric pulse to the light emitting device, thereby maintaining the mean value of light on the optical transmission line in a constant value.
In the above optical pulse transmitting method according to the first aspect of the invention, the point of detection of the symmetrical waveform signal received at the receiving side is defined to a zero-crossing point where the signal intersects the DC bias current.
According to a second aspect of the present invention, there is provided an optical pulse transmitting method in which an electric pulse is applied to a light emitting device provided at the transmitting side, thereby emitting an optical pulse from the light emitting device, which is, in turn, transmitted to the receiving side over an optical transmission line, and the transmitted optical pulse is converted into an electric pulse by a light receiving device provided at the receiving side, which is acquired as a received signal, and the method is characterized in that: in the transmitting side, a symmetrical waveform signal which changes symmetrically in positive side and negative side with a DC bias current value as its center at each of the leading edge and the trailing edge of the symmetrical waveform signal is applied as the electric pulse to the light emitting device, thereby maintaining the mean value of light on the optical transmission line in a constant value even if a pulse having long pulse width in time is transmitted over the optical transmission line.
In the above optical pulse transmitting method according to the second aspect of the invention, the detection point of reception at the receiving side is defined by the zero-crossing point of either one of symmetrical waveform signals which are generated at the leading edge and the trailing edge thereof.
In addition, a smoothing circuit is provided at the receiving side for generating a DC voltage corresponding to the DC bias current value, and the DC voltage generated from the smoothing circuit is applied to a voltage comparator as a reference voltage thereof, the voltage comparator having a hysteresis characteristic, and then a potential variation that exceeds the hysteresis width of the hysteresis characteristic with the reference voltage as its center is detected as the received signal and outputted from the voltage comparator.
In the present invention a bias current, which is constant even during no-signal period and has a value larger than the threshold value that provides a light emission starting point of the light emitting device, is applied thereto, driving it to emit a fixed quantity of light at all times. The invention proposes an optical pulse transmitting method according to which the pulse intended to send is added with a pulse opposite in polarity thereto to generate a symmetrical waveform signal which goes positive and negative symmetrically about the bias current and the symmetrical waveform signal is used to drive the light emitting device.
Furthermore, the present invention proposes an optical pulse detecting method according to which the receiving side utilizes, as a signal detection threshold value, a voltage corresponding to the bias current sent from the transmitting side.
Hence, with the use of the optical pulse transmitting method and optical pulse detecting method according to the present invention, even if the injected current vs. output light power characteristic of the light emitting device changes due to a temperature variation at the transmitting side, the bias current flowing across the light emitting device varies accordingly and the variation of the bias current is transmitted as a DC component of light to the receiving side.
Since the receiving side has a construction which regenerates the DC component of light transmitted thereto as a bias voltage and applies it as a reference voltage to a voltage comparator having a hysteresis characteristic to thereby detect rise and fall transition points of the symmetrical waveform signal, the point of detection of the symmetrical waveform signal will not change in time even if the bias voltage varies.
Thus, according to the present invention, even in the case where the light emitting device at the transmitting side undergoes a temperature change and its injected current vs. output light power characteristic varies accordingly, the point of detection of the pulse at the receiving side does not change. That is, it is possible to prevent the generation of jitter. Hence, the application of the present invention to a device of the type transmitting data over the multichannel transmission line provides an advantage that no timing error occurs in signals between the respective channels, ensuring properly timed data transmission and reception.